Surgical microscopes used in medicine, and here in particular in ophthalmology and neurosurgery should provide the means for an assistant (secondary observer) to view the same field of operation as the surgeon (primary observer). In this connection it is known to fit an independent assistant microscope on the outside of a main microscope housing. Usually, both microscopes are designed as stereo microscopes and each comprise two completely separate beam paths through which the object can be viewed from two different directions so that a stereoscopic impression is created. Both main and assistant microscope have one objective each made up of one lens or a group of lenses. The primary observer views the object or the object plane substantially vertically from above, in which the object plane runs parallel to the object plane defined by the objective or perpendicular to its optical axis. Thus the illuminated object field can be sharply imaged across its entire surface by the objective and the following eyepiece into the observer's eye.
In the case of the secondary observer the problem exists that due to reasons of space the objective assigned to him has to be arranged laterally from the primary observer's objective. So that the same object field can be imaged, the viewing direction of the secondary observer is abnormal to the object plane and runs at an angle of typically 10-30° to the normal. For one thing, this means that the object field cannot be sharply imaged as a whole and for the other that it is perspectively distorted due to the angular viewing angle.
Such aberration in the secondary observer beam path can be avoided by using microscopes where primary and secondary observer look vertically onto the object through the same objective along a mutual axis. Such microscopes are for example known from DE-C 43 31 635 (corresponds to U.S. Pat. No. 5,856,883) and DE-C 33 33 471 (corresponds to U.S. Pat. No. 4,605,287). In accordance with DE-C 43 31 635 the light coming from the object is split up between primary and secondary observer after passing the shared objective by means of a beam splitter. The secondary observer beam path is laterally uncoupled. On the microscope in accordance with DE-C 33 33 471, the beam paths are split by means of a splitting plate. In both cases a loss in light intensity is accepted. Another difficulty with such integrated devices is the complex mechanics which is necessary to be able to pivot the assistant microscope between various positions (to the right and left of the main microscope).
Assistant microscopes with their own objective to be fitted on the outside of the housing of a main microscope present an inexpensive and mechanically easy-to-do alternative to the above-mentioned integrated devices. With such devices, which have been known since the 1980s, the aberration brought about by the oblique viewing angle mentioned earlier in this document, have so far been accepted. Similar problems also exist with microscopes which for other reasons look onto an object with an objective that is arranged angular to the object plane.
The Scheimpflug principle for the sharp imaging of an object plane that is at a relative angle to the objective, which is known from cartography and photography, going back to the AT-PS 20299 from the year 1905, applies on principle for determining the position of the intermediate image plane in the case of a lens. This principle states that to attain a sharp image, the image plane is inclined in such a way that its line of intersection with the image-side principal plane of the lens is equidistant to the optical axis as the line of intersection between the object plane and the object-side principal plane. Applied to a microscope without tube lens this means that object, objective and intermediate image plane approximately intersect in a straight line. On a microscope with infinite optical system with an objective, a tube lens and approximately parallel beams in-between, the planes of the objective and the tube lens roughly correspond to those principal planes mentioned above.